JP7237961B2 - Method and apparatus for post-weld heat treatment of aluminum alloy parts and welded aluminum parts treated according to the method - Google Patents

Description

The present invention relates to a method and apparatus for post-weld heat treatment of welded aluminum alloy parts and welded aluminum alloy parts treated according to the method.

For example, the low density of aluminum alloys compared to steel provides a high strength-to-weight ratio. This makes aluminum alloys attractive in many structural applications, such as in the automotive industry, marine and offshore structures, bridges and buildings. However, welded aluminum alloys experience a significant loss of strength due to the formation of "softened zones" by the welding process. This problem presents a serious limitation to the use of structural aluminum due to the significantly lower load-bearing capacity in the weld zone compared to the unaffected base metal.

In current design standards for aluminum alloys, such as Eurocode 9, this strength reduction is accounted for by the introduction of a strength reduction factor. These factors can be as low as 0.5, meaning that only 50% of the matrix strength is available. The actual modulus will depend on the alloy type and processing conditions. Therefore, innovative welding solutions are needed to fully exploit the strength of structural aluminum.

The present invention presents a possible solution to the strength loss problem associated with welding. The present invention is useful for welding processes such as metal inert gas (MIG), tungsten inert gas (TIG), laser and hybrid processes (e.g. laser and MIG), cold metal transfer (CMT) and friction stir welding (FSW) processes. It can be applied to several types of welding processes, including fusion welding processes. The present invention provides a new and novel method and apparatus for optimizing the load-bearing capacity of welded aluminum alloy structures by localized post-weld heat treatment (PWHT).

The method involves post-weld heat treatment of a welded aluminum alloy component having a heat affected zone with reduced load-bearing capacity, wherein the heat affected zone is identified and a heating source comprises at least one of said heat affected zones. A heating source is applied to two first locations to generate a temperature above T _min and the heating source is held at said location for at least a time t _min .

The apparatus includes a heat source movable relative to the aluminum alloy part and further capable of being positioned at a predetermined position on the part, the heat source being transmitted to the part at said position. Further control is possible with respect to temperature and dwell time which affect the heat generated.

Different methods of localized heating can be used, including induction heating, laser heating, electrical resistance heating, friction stir welding tools, and the like. This concept can be used for different alloy systems, including the 4xxx, 6xxx and 7xxx series of age-hardened alloys and especially the 5xxx series of work-hardened alloys. The potential strength gains and concomitant weight savings are particularly significant in 6xxx alloys due to the large strength loss in the heat affected zone (HAZ) in these types of alloys. Weight reduction is not only beneficial in terms of weight reduction of structures, but is also directly related to material costs.

Different types of aluminum products or parts can be used, including extruded profiles, sheet material made by rolled and cast alloys, and combinations thereof.

This local post-weld heat treatment can significantly increase the load-bearing capacity of the component.

These and further advantages can be achieved by the invention as defined in the appended claims.

The invention is further illustrated by examples and figures.

FIG. 4 graphically illustrates the results of hardness measurements across a weld of a 6060 type alloy; FIG. FIG. 4 illustrates heat affected zones on both sides of a longitudinal weld without localized PWHT. FIG. 4 illustrates the heat affected zones on both sides of a longitudinal weld after local PWHT; FIG. FIG. 2 illustrates the load-bearing capacity _F1 of the weld shown in FIG. 4 illustrates the load carrying capacity _F2 of the weld shown in FIG. 3 subjected to localized PWHT. FIG. 11 illustrates how a heat source for localized PWHT can manipulate the location of the weakened area. FIG. Figure 3 illustrates a pattern by which the heating source can be moved in local PWHT. Figure 6 illustrates how the location of the vulnerable area can be manipulated in a controlled manner. Figure 4 illustrates the use of a second localized heat treatment; A theoretical setup is disclosed for visualizing the effect of PWHT according to the present invention. A validation set-up for the effectiveness of rapid PWHT in the HAZ is disclosed with linear and wavy shapes. Visualize the effective stress in the center of a 2 mm thick plate for a HAZ yield stress of 115 MPa in a straight HAZ. Visualize the effective stress in the middle of a 2 mm thick plate for a HAZ yield stress of 115 MPa in a bulging HAZ. 12 is a table showing a summary of simulations based on the samples of FIG. 11; Further examples of areas of weakness after localized post heat treatment are disclosed. Discloses a cross-section of the welded parts subjected to a force in the transverse direction of the weld. Discloses a cross-section of the welded parts under pressure perpendicular to the surfaces of the parts. The strain distribution during load application across the weld without PWHT is shown as different grayscales. 19 shows the location of the weld of FIG. 18 and an indication of the location of the fracture corresponding to the location of the softened zone in the heat affected zone without PWHT. Similar to FIG. 18 and showing the strain pattern in grayscale, but here local PWHT is applied for transverse heating according to the present invention. Figure 20 shows the local PWHT signature of Figure 20; Disclosed are the recorded stress versus elongation for the two conditions described in FIGS. 18-19 and 20-21, respectively.

FIG. 1 illustrates the results of hardness measurements across a weld 11 of a 6060 type alloy, illustrating the problem addressed by the present invention. The softened zone from the weld to the boundaries 12, 13 in the HAZ reduces the load bearing capacity. Hardness measurements across the weld reveal these areas of softening.

FIG. 2 illustrates a heat affected zone with boundaries 12, 13 on either side of the longitudinal weld 11 as shown in FIG. This is the point of weakness in the prior art.

FIG. 3 illustrates the location of the heat affected zone on either side of the longitudinal weld 11 after localized PWHT according to the present invention. Due to the selected local post-weld heat treatment (PWHT), the boundaries 22, 23 of the heat affected zone are illustrated here as a zigzag pattern.

FIG. 4 illustrates the load carrying capacity _F1 of the weld 11 shown in FIG.

FIG. 5 illustrates the load carrying capacity _F2 of weld 11 shown in FIG.

It can be demonstrated that this local PWHT results in significantly higher cross-weld load carrying capacity (ie, F ₂ >>F ₁ ).

This is due to the fact that the larger areas of weakness are adapted to distribute forces. In some areas the zone of weakness is parallel to the direction of load application.

The location of the weak zone can be manipulated as follows. A heating source (eg, an induction coil) is moved along a predetermined pattern. This pattern can be a simple pattern, for example straight lines as shown in the left part of FIG. In this example, the heating source is first moved to position 1 and turned on. The power is then turned off, the heating source is moved to position 2, where the power is turned on again, and so on. This creates a new vulnerable area pattern, as shown in the right figure, where the actual pattern 32 (rightmost) deviates slightly from the ideal rectangular zigzag pattern 22 . The weld is designated by reference number 11 .

The pattern along which the heating source moves can be complex and orthogonal or angled to the weld. The pattern can also be curved in shape, as illustrated in FIG. 7 (see, eg, reference number 33). These patterns can also traverse the weld 11 once or several times. It should be appreciated that the heating source may be turned on during movement following such patterns and turned off during movement between heat-affecting patterns.

において説明されているように、物理ベースの材料モデルに入力される。 The shape (including width) and location of the heat source pattern, and the intensity (i.e., power), which can vary and be a function of position, can be pre-defined by different tools such as FE code combinations to calculate the weld thermal cycle. can be calculated, then for example

input into a physics-based material model, as described in

The modeling concepts described above can also be used in combination with optimization tools. A shallow neural network or similar software tool can be used to determine the optimal location, shape and power of the heating source pattern.

を参照されたい）。６ｘｘｘ－Ｔ６アルミニウム合金では、ＨＡＺの最脆弱域は、通常、図に線（脆弱域の元の位置）で表すように、４００℃の等温線の近傍に位置する。図に表されるおよその位置で表面に加熱源を適用することにより、ＨＡＺが再加熱される。この局所熱処理中に達した最高温度の等温線が白線で図示されている。これらの等温線は、同様のアルミニウム構造体に関する前回のシミュレーションに基づく大まかな推定値である。図に示すように、ここで、４００℃の等温線の白線が溶接部中心線から更に離れた位置まで移動している。溶接部の最脆弱域がこの位置と厳密に一致する。 Figure 8 illustrates how the location of the vulnerable area can be moved in a controlled manner. FIG. 8 discloses a section perpendicular to the welding direction. The starting point is aluminum melt welded to a 12.5 mm thick aluminum plate. Peak temperatures are shown as areas of different gray scales. The corresponding temperatures are marked by the scale bar on the left (for details see

(see ). For 6xxx-T6 aluminum alloys, the zone of greatest weakness in the HAZ is usually located near the 400° C. isotherm, as represented by the line (original position of the zone of weakness) in the figure. The HAZ is reheated by applying a heat source to the surface at approximately the location represented in the figure. The maximum temperature isotherm reached during this localized heat treatment is illustrated by the white line. These isotherms are rough estimates based on previous simulations on similar aluminum structures. As shown in the figure, the white line of the 400° C. isotherm has now moved further away from the weld centerline. The weakest zone of the weld coincides exactly with this position.

As explained above, it is not only possible to move the position of the vulnerable area to expand the vulnerable area. By using a second localized heat treatment after the first localized heat treatment, artificial age hardening can be obtained in regions where the temperature exceeded about 460-480° C. in the first localized heating cycle (see FIG. 9). sea bream).

A full solution heat treatment may require temperatures in excess of 520° C., depending on the alloy composition and how the alloy is worked. Initial temperature conditions are of particular importance. Hardened particles (i.e. clusters at T4 conditions) are smaller at T4 compared to T6 or T7, so T4 conditions require a lower temperature compared to T6 or T7 to bring Mg and Si into solid solution .

However, the "partial" solution heat treatment, which corresponds in part to the second aging cycle, is performed at a lower temperature, up to about 460-480°C.

The right part of FIG. 9 illustrates a second localized heating in which the temperature is maintained at about 180-250° C. for some time. Yield strength increases significantly with actual temperature cycling at each location. The position (ie pattern) followed by the heating source and the power applied are typically different in the second heating cycle compared to the first heating cycle.

Starting with the heat treatment according to the invention as described with respect to FIG. 5, reference is made to FIG. 10, which shows a top view of the welded plate halves, in which the vertical symmetry line along the weld is indicated. Here, position 0 indicates the weld metal, 1 indicates the _T4 zone, and positions 2 and 4 indicate the outer boundaries of the HAZ after the welding operation and subsequent heat treatment. The "fingers" at position 3 represent areas of the HAZ that have been heat treated to withstand loads similar to those of the T4 area described above. Position 5 represents the T6 zone where the load bearing properties are unaffected by the welding operation.

Based on the lengths L1, L2, L3 and L disclosed in the figure, the ultimate tensile strength (UTS) at positions 0-5 is as follows:
0. UTS_WELD METAL 1. UTS_T4
2. ((L1+L2) ^* UTS_HAZ+L3 ^* UTS_T4)/L
3. (L1 ^* UTS_T6+L2 ^* UTS_HAZ+L3 ^* UTS_T4)/L
4. (L1 ^* UTS_T6+(L2+L3) ^* UTS_HAZ)/L
5. UTS_T6
can be set.

The numerical example below shows how the above relationship can be used to estimate the effect of PWHT application on the resulting increase in load-bearing capacity.

Examples: L=200 mm, L1=45 mm, L2=5 mm, L3=150 mm, UTS_T4=200 MPa, UTS_HAZ=150 MPa, UTS_T6=300 MPa.

From the above relationships, we find the following values for ultimate tensile strength (UTS) at positions 1-5:
1. UTS = 200MPa
2. UTS = 187.5 MPa
3. UTS = 221.3 MPa
4. UTS = 183.8 MPa
5. UTS = 300MPa
get

Therefore, in this example, the minimum UTS of the component for load bearing capacity is 183.8 MPa. The corresponding load-bearing capacity for welded parts without PWHT is 150 MPa. Therefore, the predicted increase in load-bearing capacity due to PWHT is 22.3%.

By subjecting zone 1 to a separate heat treatment, it may be possible to increase the ultimate tensile strength (UTS) in this zone. Zone 1 in FIG. 9 corresponds to the softened zone in the HAZ shown in FIG. By performing an optimum post-weld heat treatment in this region, the strength of the material can be increased to a strength similar to that of T6. Application of the localized PWHT method described above can also be used to increase the strength of the weld metal, ie region 0 in FIG. The possible strength enhancement of the weld metal is given by the composition of the base metal and filler wire respectively, the resulting chemical composition in this region and the so-called "dilution" which defines the relative proportions of filler wire and base metal in the weld metal. is determined by

Simulations investigated the effect of rapid PWHT treatment in leading to a significant enhancement of the fully grain dissolved zone compared to the HAZ zone of minimal intensity. In FIG. 11, 4 samples based on 2 mm thickness and 4 samples based on 5 mm thickness are given. In each of these groups there are samples with two different yield stress values in the minimum strength HAZ zone (115 MPa and 125 MPa) and also a linear HAZ and a wavy HAZ. A wavy HAZ is caused by local induction heating.

In FIG. 12, the effective stress in the middle of a 2 mm thick plate is visualized for a HAZ yield stress of 115 MPa in a straight HAZ.

FIG. 13 visualizes the effective stress in the center of a 2 mm thick plate for a HAZ yield stress of 115 MPa in a bulging HAZ.

Visualizations similar to those shown in FIGS. 12 and 13 have been performed for all eight samples.

FIG. 14 discloses a simulation overview based on the sample of FIG. The figure shows that for the straight HAZ profile, the transverse strength is limited by the HAZ strength, whereas for the wavy HAZ profile, severe local yielding does not occur unless a much higher transverse loading stress is applied, thus reducing the overall load bearing capacity. It clearly illustrates that the force can be greatly enhanced. This result also indicates good energy absorption, since the transverse elongation is about 50% greater for the same value of maximum local strain.

For example, a comparison of Samples 111 and 121, both involving a 2 mm thick plate but having straight and wavy HAZ geometries respectively, shows that the simulated stress load in the transverse direction increased from 189 MPa to 234 MPa. .

This simulation confirms that the strength of welded aluminum parts can be enhanced by modification of the HAZ geometry. These examples confirm that the shape of the remaining matrix should preferably be a straight thin finger to the softened zone rather than a zig-zag or blunt shape. It has been shown that the greater the width of the HAZ relative to the plate thickness, the greater the strength improvement. The effect is believed to be stronger when PWHT is applied to enhance the intensity of the inner (T4) region.

FIG. 15 shows examples of locations of weakened zones 22', 23' after local post-weld heat treatment that can be applied to different loading conditions. The locations of the weakened zones after the welding operation are designated 12', 13'. The actual loading force may be transverse or parallel to the weld (shear forces acting in opposite directions on each side of the weld 11), or a combination thereof. Forces can also act in-plane or out-of-plane. The force can be distributed or act as a concentrated load.

This force can also act due to pressure applied perpendicular to the surface of the part or product. In addition, such loads can be blast loads acting on the part or product at high velocities.

FIG. 16 discloses a cross-section of the welded parts subjected to a force transverse to the weld 11. FIG.

FIG. 17 discloses a cross-section of the welded parts under pressure perpendicular to the surfaces of the parts. The weld is disclosed at 11'.

Experimental validation of the concept:
FIG. 18 shows the strain distribution during load application across the weld when no local PWHT is applied. The principal stress during loading across the weld was obtained by digital image correlation (DIC) when no transverse heating (localized PWHT) was applied.

In this experimental device, welding was performed by MIG welding. However, similar stress patterns will occur using other welding techniques, such as when the weld is performed by friction stir welding.

In the figure, the strain distribution is shown in different gray scales. From this figure it is evident that the strain builds up along two lines parallel to the weld: the white areas closely along the heat affected zone (HAZ) located on either side of the weld. . This is the normal condition during load application transverse to the weld direction when no localized heating is applied, ie, no PWHT.

FIG. 19 discloses the location of the weld of FIG. 18 and an indication of the location of the fracture corresponding to the location of the softened zone in the heat affected zone.

FIG. 20 discloses the strain distribution during load application across the weld when localized PWHT is applied. FIG. 21 discloses the location of the weld and an indication of the location of the applied local PWHT pattern. The location of the break is also indicated.

Figures 20 and 21 are similar to Figures 18 and 19, respectively, except that localized PWHT was applied for transverse heating by the friction stir source. However, any suitable heating source, such as a laser, could have been applied for this localized PWHT. The resulting strain pattern, shown in FIG. 20, differs significantly from that of FIG. 18 because the strain results in a nearly regular pattern. FIG. 21 also shows local PWHT signatures and locations of MIG welds and fracture locations.

FIG. 22 shows the recorded stress versus elongation for the two different cases described above: no application of a localized heat source (dashed line) and application of a localized heat source across the weld according to the invention (solid line). ing.

Different strain patterns shown in FIGS. 18 and 20 result in different responses during transverse loading, as shown in FIG. From this figure, it is clear that samples with localized PWHT patterns exhibit better overall performance than those without. Therefore, both the maximum stress and the elongation to break were better for the samples with localized PWHT according to the invention compared to those without.

In practice, the design and placement of the heat effect pattern must be optimized for the actual design load, and it should be understood that it may differ for different aluminum alloys and different combinations of multi-material solutions.

Additionally, the heating source can be moved in any configuration that yields results according to the present invention. For example, the heating source can be moved in a basic circular pattern that can be combined with propagating movement.

Claims (19)

1. A method for post-weld heat treatment of welded aluminum alloy parts, the weld having an extension (e) having a heat affected zone of reduced load-bearing capacity, said method comprising:
- identifying the heat affected zone ;
- applying a heating source to at least one first location of said heat affected zone;
- said heating source generates a temperature above Tmin ;
- said heating source is held at said location for at least a time tmin;
- the heating source is removed from the first point after a time tmin and applied to a second point along the extension of the weld at a predetermined distance from the first point; step and
characterized by a local post-weld heat treatment with
The method , wherein the local post-weld heat treatment enlarges the area of the heat affected zone for improved force distribution across the weld. 2. A method according to claim 1, characterized in that after a time tmin the heating source is moved in contact with the aluminum alloy part. 3. A method according to claim 1 or 2, characterized in that the heating source is moved in a direction transverse to the heat affected zone. A method according to any one of claims 1 to 3, characterized in that the heating source is moved in a rectangular zigzag pattern. A method according to any one of the preceding claims, characterized in that the heating source is moved according to pre-calculated lines and curves so as to form the enlarged heat affected zone . 2. The method of claim 1, wherein the weld is treated with the local post-weld heat treatment . 2. The method of claim 1, wherein after the post local weld heat treatment , the aluminum alloy component is heat treated in an annealing furnace. Apparatus for post-weld heat treatment of welded aluminum alloy parts having a heat affected zone with reduced load-bearing capacity according to the method according to claims 1-7, wherein the weld zone comprises an extension (e) has
The apparatus includes a heat source movable relative to the component and further positionable at a predetermined location on the component along the weld, the heat source comprising: further controllable with respect to the temperature affecting the heat transferred to the component at the location and the time the heating source is held at the predetermined location on the component ;
The apparatus of claim 1, wherein the local post-weld heat treatment progressively enlarges the area of the heat affected zone along the weld for improved force distribution across the weld. 9. The apparatus of claim 8, wherein the heat source is attached to welding equipment that moves along the part. 9. Apparatus according to claim 8, characterized in that the heating source does not move while the part is being moved. 9. Apparatus according to claim 8, characterized in that the heating source is controlled by a programmable PLC. 9. Apparatus according to claim 8, characterized in that the heating source is attached to a manipulator or robot controlled by a programmable PLC. In a welded aluminum alloy component having a heat affected zone treated according to the local post-weld heat treatment according to any one of claims 1 to 7,
The local post-weld heat treatment causes the area of the heat affected zone along the weld to be progressively enlarged for improved force distribution across the weld, thereby enhancing the load-bearing properties of the component. A welded aluminum alloy component, characterized in that it provides: 14. The heat affected zone of claim 13, wherein additional regions of the heat affected zone along the weld due to the local post-weld heat treatment have an orientation different from the orientation of the main direction of the weld. with welded aluminum alloy parts. The additional area of the heat affected zone due to the local post-weld heat treatment along the weld increases the load-bearing capacity of the heat affected zone by increasing the material's ability to withstand shear forces. 15. A welded aluminum alloy part with a heat affected zone according to claim 13 or 14, characterized in that it is oriented so as to increase the . Having a heat affected zone according to any one of claims 13 to 15, characterized in that the additional area of the heat affected zone due to the local post-weld heat treatment along the weld has a zigzag pattern. Welded aluminum alloy parts. A welded aluminum alloy part with a heat affected zone according to any one of claims 13 to 16 , characterized in that it comprises at least one of an extruded part, a rolled part or a cast part. Welded aluminum alloy part with a heat affected zone according to any one of claims 13 to 17 , characterized in that it is welded to a part of a metallic material other than aluminum or an aluminum alloy. 19. Welded aluminum alloy part with heat affected zone according to claim 18 , characterized in that it is welded to a steel part or a steel alloy part.

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